Composite materials are made of axially strong, stiff reinforcing fibers that are embedded in a matrix of a tough resin. The fibers most widely used in composites are carbon, glass, aramid (Kevlar.RTM.), and boron. These fibers can be made into thin, flat ribbons or long narrow strands which can be weaved or stitched to form a reinforcing fabric.
The resins are typically thermosets or thermoplastics. Thermosets include, for example, epoxies, polyesters, and polyimides. Thermosets are cured at an elevated temperature and cannot be reshaped by reheating. Thermoplastics include, for example, styrene, acrylics, cellulosics, polyethylenes, vinyls, nylons, and fluorocarbons. Thermoplastics can be reshaped by raising them above their glass transition temperature. This property limits thermoplastics to use in lower temperature environments only.
Composite materials can exhibit high strength, resistance to creep, fatigue, and chemical attack. But improper curing can result in unreliable composite parts.